For the purpose of this text, “purified water” means generally water of greater purity than ordinary potable water available from e.g. a municipal water distribution system. Highly purified water is required e.g. for various medical purposes, such as production of pharmaceuticals, and in the production of “water for injection”, the quality of which is defined in official pharmacopoeias. Plants for producing such purified water may be designed as so-called multi-effect stills, employing a series of falling film evaporators. A falling film evaporator comprises a vertical bundle of evaporation tubes enclosed into a heating jacket. Water fed into the evaporating tubes at their upper end flows down the inner surface of the tubes, thereby evaporating and forming steam, which emerges at the lower ends of the tube bundle together with any un-vaporized water. In many devices for producing highly purified steam or water, the flow of steam makes a 180° turn and flows upwards in a rising space provided therefore, while the remaining water collects at the bottom of the device.
The upward-flowing steam may, depending on the type of device, undergo various kinds of demisting and/or purification operations, whereby the separated droplets and impurities finally join the water phase at the bottom of the effect while the steam enters the heating jacket of the subsequent effect. The water phase becomes the feed water of the subsequent effect and the steam condenses, giving off its heat to cause more steam to evaporate from the feed water, and the cascade continues until the last effect is reached. The combined condensates from the heating jackets, together with the steam from the last effect, are cooled and condensed, respectively, in a final heat exchanger to form the purified product water.
In prior art multi-effect falling-film plants for producing purified water, the feed to each effect generates a steam phase and a residual leaving water phase. The residual water phase then, in its entirety, generates the feed for the next effect and so on, until the last effect is reached. The residual water in the last effect represents the reject fraction containing the impurities.
In this context, a reject stream is a stream of water, which is permanently removed from the process. Generally, the reject stream is liquid water, but maybe a steam-liquid mixture. A continuous process for producing pure water by evaporation must include the removal of a sufficient amount of reject water to carry away the separated impurities. The ratio of reject water to product water is an important aspect of the efficiency of the process. It follows from the material balance, that if the reject stream is insufficient, the impurities either accumulate in the device or leave with the product, which both alternatives are highly undesirable.
In Finnish patent application 20021538, a unit for producing pure steam is disclosed. This device includes a particular purification section for the steam, having a rising spiral path following the evaporation section, which is a falling-film unit as described above. In the outer wall of the spiral path, apertures are provided, and outside said apertures a cool surface. This arrangement creates a pressure gradient across the apertures due to condensation of steam on the cool surface, causing impurity-containing droplets to migrate through the apertures and collect on the cool surface. The least pure component of the rising stream in the spiral path thus travels to the periphery and leaves through the apertures, condenses on the cool surface and forms a water phase rich in impurities, which flows down the cool surface. This water phase rich in impurities may be isolated and not returned to the process, but separated as a reject stream.
In Japanese patent 10328501, a two-effect falling film evaporator is disclosed having a centrifugal demister adapted to the second effect. The steam phase from the first effect enters the demister, droplets are separated and form a stream, which can be discharged or conducted to the pipe transferring the liquid phase from the first to the second effect. The demisted steam phase enters the heating side of the second effect.